Those skilled in the art know that drive is provided for a vehicle such as a class 8 truck from a forward rear drive axle to a rear rear drive axle of tandem axles through an inter-axle driveline. Typical tandem axles have high inter-axle driveline cardan joint angles due to the high position of the forward axle output shaft joint and the low position of the rear axle input joint. High inter-axle driveline cardan joint angles in typical tandem axles can also be attributed to the short distance between the forward axle output joint and the rear tandem axle input joint. Those skilled in the art know that high inter-axle driveline carden joint angles are generally undesirable since the noise and vibration of the joints increase as the angles increase. The low position of the rear axle input joint also undesirably reduces the ground clearance of the inter-axle driveline.
Based on the above, it can be appreciated that a long inter-axle driveline is desirable since such a driveline will reduce the angles. Therefore, if the standout dimension, which is the distance between the centerline of the axle shaft and the front of the rear axle input shaft, can be reduced, a longer inter-axle driveline can be accommodated in the tandem axle system.
Various prior art inventions have tried to address these disadvantages of tandem axles. For example, U.S. Pat. No. 1,856,748 (hereinafter “the '748 patent”) provides for a driving mechanism designed to eliminate excessive angles in the universal joints of vehicles under normal driving conditions. FIG. 2 of the '748 patent depicts a propeller shaft e driving a universal joint f. Joint f is connected to a first differential mechanism f1. The joint f supplies power to f1 which apportions that power between axles b1 and c1. A hyperbolical spiral hypoid driving pinion f2 supplies a portion of the power to the ring gear b4 while a second similar driving hypoid pinion f3 supplies the remaining power to the ring gear c4. It should be noted that both the f2 and f3 driving hypoid pinions are operating on the coast side of the respective b4 and c4 ring gears which is known to be the undesirable weak side of the gear tooth in the '748 patent.
A shaft g connects the differential housings b3 and c3. To align the shaft g with propeller shaft e, the axis of the forward driving pinion f2 falls above the axis of the shaft b1 while the axis of pinion f3 is below the axis of shaft c1. It should be noted that the rear axle input is below center and as such does not provide good ground clearance for the rear of inter-axle driveline g.
According to the '748 patent, this design aligns the axis of pinions f2 and f3 with one another and with the shafts e and g. The pinion f3 drives from the rear side of the ring gear c4 and the forward pinion f2 drives from the forward side of the ring gear b4.
U.S. Pat. No. 1,791,138 (hereinafter “the '138 patent”) provides for a dual axle drive having ring gears f1 and f3 mounted on opposite sides of the transmission shaft x, as best seen in FIG. 6. The hypoid pinion f meshes with the ring gear f1 rearward from the live axle a3 while the hypoid pinion f2 meshes with the ring gear f3 forward of the live axle b3.
FIG. 3 of the '138 patent depicts the forward pinion on the rear side of the forward ring gear and the rear pinion on the forward side of the rear ring gear. The '138 patent also teaches that the rear pinion is located above the center of the rear ring gear. The forward pinion is also above the center of the front ring gear. It should be noted that the forward drive hypoid pinion f is operating on the desirable stronger side of ring gear f1 but the rear drive hypoid pinion f2 is operating on the undesirable weak coast side of ring gear f3. Additionally, the placement of the inter-axle power divider differential d1, d2, d3 components and the forward axle pinion f to the rear of the forward axle results in an undesirably short inter-axle driveline.
Despite trying to address some of the problems with tandem axles, the representative prior art discussed above can be improved. Specifically, it would be advantageous to optimize the inter-axle driveline by minimizing the cardan joint angles and improving the inter-axle driveline ground clearance.